Hitherto, a plurality of dose rate measuring devices having sensitivity and accuracy depending on the level of radiation to be measured have been installed in the vicinities of facilities such as a nuclear facility and a spent nuclear fuel reprocessing facility, in order to measure a dose rate over a wide range from a natural, radiation level to a high radiation level during an accident.
However, in a case where the plurality of dose rate measuring devices are installed close to each other, the devices may act as obstacles of radiation incidence with respect to each other. In addition, the plurality of dose rate measuring devices are prepared, and thus a problem of an increase in device cost occurs. For this reason, it is required to perform measurement corresponding to a wide range of close rate using a single dose rate measuring device,
For such a request, PTL 1 discloses a dose rate measuring device in which a thallium-activated sodium iodide scintillation detector is provided with a lead shield, and methods of measuring a dose rate in accordance with the level of a dose rate in a measurement field are automatically switched.
In such a dose rate measuring device, a pulse height discrimination bias modulation (DBM) method is adopted for the measurement of a dose rate in a low dose rate region (hereinafter, called a low-range dose rate), and a current measurement method is adopted for the measurement of a dose rate in a high dose rate region (hereinafter, called a high-range dose rate).
The DBM method is to convert a current pulse which is output by a scintillation detector into an analog voltage pulse to amplify the converted pulse and remove high-frequency noise, to weight the pulse with a dose rate in a DBM circuit to set the weighted pulse to a pulse of a repetitive frequency proportional to the dose rate, and to calculate the low-range dose rate on the basis of this pulse.
In addition, the current measurement method is to input a direct current which is output by a scintillation detector to a voltage/frequency converter to set the input current to a pulse of a repetitive frequency proportional to a voltage, and to calculate the high-range dose rate on the basis of this pulse.
Energy characteristics as an error generated depending on energy of a γ-ray are different from each other in the low-range dose rate and the high-range dose rate, and optimum switching points of the low-range dose rate and the high-range dose rate are different from each other depending on energy of γ-ray. Therefore, in a case where two measurement methods are switched with a fixed dose rate, a stepped difference between output energy characteristics occurs in the switching point. On the other hand, in PTL 1, the detector is provided with the lead shield, and thus the stepped difference is reduced.